The purposes of the study are (1) Assess the effectiveness of chronic electrical stimulation of midbrain sites for the relief of chronic pain in humans; (2) Evaluate the efficacy and mechanisms of traditional narcotic analgesia and compare these to chronic electrical stimulation of midbrain sites; (3) Validate experimental models of pain and their potential diagnostic use in chronic pain patients; and (4) Determine and compare the impact of both traditional narcotic and chronic electrical stimulation therapies on the functional, intellectual and emotional well being of these patients. Participants in this study will be (1) chronic pain patients receiving surgically implanted stimulating electrodes for pain control; (2) chronic pain patients maintained on traditional narcotic analgesics who will not receive implanted stimulating electrodes; and (3) healthy normal volunteers. The effects of chronic brain stimulation in surgical patients will be compared to the effects of narcotics previously administered to patients and to effects of narcotic regimes in nonsurgical chronic pain patients. In addition, the effects of narcotics on perceptual and neural mechanisms of experimentally induced pain will be assessed in pain-free volunteers. Results from 16 patients this year support previous findings from this project. Morphine significantly decreased the magnitude of low back pain and this effect was reversed by naloxone. The effect of deep brain stimulation was no greater than that observed after sham stimulation and this effect was not reversed by naloxone. The thermal pain studies showed that morphine altered pain sensations mediated by both A-delta and C-fiber primary afferents, and in addition, attenuated the central C-fiber summation observed with delivery of repetitive stimuli delivered at fast rates.